This invention relates to a method of manufacturing a powder of Bi-based superconductive oxide containing lead, and a method of manufacturing a sintered body having high density and excellent superconductivity therefrom.
It is conventionally known that a Bi--Sr--Ca--Cu--O oxide (hereinafter referred to as "Bi-based oxide" unless otherwise specified) formed of bismuth (Bi), strontium (Sr), calcium (Ca), copper (Cu), and oxide (O) exhibits superconductivity at a temperature equal to or higher than the boiling point of liquid nitrogen. However, the known Bi-based oxide is formed by two coexisting phases, i.e., a low temperature phase having a critical temperature (Tc) of 75.degree. K. at which it exhibits superconductivity (hereinafter merely called "critical temperature") and a high temperature phase having a critical temperature of 105.degree. K. It has been difficult to form a superconductive oxide having the high temperature phase alone.
Under the above circumstances, it has recently been found that a Bi--Pb--Sr--Ca--Cu--O oxide (hereinafter referred to as "Bi-based oxide containing Pb" unless otherwise specified) having a high temperature phase (Tc=105.degree. K.) alone can be stably formed by replacing part of Bi by lead (Pb) (see "Abstract of Spring Meeting in 1988" published by "Japan Society of Powder and Powder Metallurgy", page 63). This Bi-based oxide containing Pb is formed by a coprecipitation method or a solid phase reaction method. The coprecipitation method comprises forming coprecipitates in a solution of nitrate or oxalate of Bi, Pb, Sr, Ca, and Cu, thereby obtaining a powder of Bi-based oxide containing Pb. On the other hand, the solid phase reaction method comprises sintering a mixed powder of a Bi oxide powder, a Pb oxide powder, a Ca carbonate powder, a Sr carbonate powder, and a Cu oxide powder at a predetermined temperature into a composite sintered body, and crushing the composite sintered body into a powder of Bi-based oxide containing Pb.
The powder of Bi-based oxide containing Pb thus prepared is pressed into a green compact. The green compact is then sintered at a temperature within a range from 800.degree. to 900.degree. C. under atmospheric pressure, thereby obtaining a sintered body of the Bi-based oxide containing Pb. Alternatively, a sintered body may also be obtained by hot pressing the powder of Bi-based oxide containing Pb at a temperature within a range from 650.degree. to 850.degree. C.
The Bi-based oxide powder obtained by the coprecipitation method has a structure having a high dispersion of elements. Particularly Pb is highly dispersed in Bi such that part of the lattice points of Bi atoms are replaced by Pb atoms evenly throughout the lattice structure. Therefore, the Bi-based oxide obtained by the coprecipitation method has a high temperature phase alone. However, the coprecipitation method takes a long time to form coprecipitates so that it is not suitable for production of a powder of Bi-based superconductive oxide on a commercial scale.
On the other hand, the solid phase reaction method is suitable for mass production of superconductive oxide powder, but has the following disadvantages:
(1) Since the starting powders in the form of solid particles are mixed and sintered, the resulting sintered body has a low degree of dispersion of Pb in Bi, i.e., Bi atoms in the lattice are not replaced by Pb atoms evenly throughout the lattice structure, thus making it difficult to obtain a powder of superconductive oxide having a high temperature phase alone.
(2) Since Pb oxide (PbO) and Bi oxide (Bi.sub.2 O.sub.3) have low melting points, i.e., approximately 900.degree. C. and 820.degree. C., respectively, sintering must be carried out at a temperature lower than 820.degree. C. in order to prevent melting of these oxides during sintering. However, due to such a low sintering temperature, the resulting sintered body has a degraded superconductivity, a particularly insufficient critical electric current density (Jc), because residual carbon is present in grain boundaries due to decomposition of the Ca carbonate and Sr carbonate of the mixed powders.
(3) There occurs sublimation of a small part of the Pb oxide even during sintering of the mixed powder containing the Pb oxide at a temperature slightly lower than 820.degree. C., so that the resulting sintered body has too low a Pb content to obtain a superconductive oxide sintered body having a high temperature phase alone.
(4) Even if sintering is carried out at the highest possible temperature, i.e., 900.degree. C., the resulting sintered body will have a relative density of 75% at most. Further, the sintering at such a high temperature under atmospheric pressure causes deficiency of oxygen in the resulting sintered body and hence spoils the superconductivity. To recover oxygen, the sintered body has to be subjected to oxygen annealing at a low temperature over a long period of time.
On the other hand, if hot pressing is carried out, the resulting sintered body will have a higher relative density of 90 % or more. However, the hot pressing also causes deficiency of oxygen in the sintered body, resulting in degraded superconductivity. To recover oxygen, the sintered body has to be subjected to oxygen annealing for about 100 hours.
The oxygen deficiency can be avoided by atmospheric pressure sintering or hot pressing at a lower temperature than that in the conventional method. However, the resulting sintered body has a low relative density and hence a low critical electric current density, thus failing to exhibit satisfactory superconductivity.